events.rst

.. index::
   single: event
   single: subscriber
   single: INewRequest
   single: INewResponse
   single: NewRequest
   single: NewResponse

Using Events

An event is an object broadcast by the :app:`Pyramid` framework at interesting points during the lifetime of an application. You don't need to use events in order to create most :app:`Pyramid` applications, but they can be useful when you want to perform slightly advanced operations. For example, subscribing to an event can allow you to run some code as the result of every new request.

Events in :app:`Pyramid` are always broadcast by the framework. However, they only become useful when you register a subscriber. A subscriber is a function that accepts a single argument named event:

def mysubscriber(event):
    print event

The above is a subscriber that simply prints the event to the console when it's called.

The mere existence of a subscriber function, however, is not sufficient to arrange for it to be called. To arrange for the subscriber to be called, you'll need to use the :meth:`pyramid.config.Configurator.add_subscriber` method or you'll need to use the :func:`pyramid.events.subscriber` decorator to decorate a function found via a :term:`scan`.

Configuring an Event Listener Imperatively

You can imperatively configure a subscriber function to be called for some event type via the :meth:`~pyramid.config.Configurator.add_subscriber` method (see also :term:`Configurator`):

from pyramid.events import NewRequest

from subscribers import mysubscriber

# "config" below is assumed to be an instance of a
# pyramid.config.Configurator object

config.add_subscriber(mysubscriber, NewRequest)

The first argument to :meth:`~pyramid.config.Configurator.add_subscriber` is the subscriber function (or a :term:`dotted Python name` which refers to a subscriber callable); the second argument is the event type.

Configuring an Event Listener Using a Decorator

You can configure a subscriber function to be called for some event type via the :func:`pyramid.events.subscriber` function.

from pyramid.events import NewRequest
from pyramid.events import subscriber

@subscriber(NewRequest)
def mysubscriber(event):
        event.request.foo = 1

When the :func:`~pyramid.events.subscriber` decorator is used a :term:`scan` must be performed against the package containing the decorated function for the decorator to have any effect.

Either of the above registration examples implies that every time the :app:`Pyramid` framework emits an event object that supplies an :class:`pyramid.events.NewRequest` interface, the mysubscriber function will be called with an event object.

As you can see, a subscription is made in terms of a class (such as :class:`pyramid.events.NewResponse`). The event object sent to a subscriber will always be an object that possesses an :term:`interface`. For :class:`pyramid.events.NewResponse`, that interface is :class:`pyramid.interfaces.INewResponse`. The interface documentation provides information about available attributes and methods of the event objects.

The return value of a subscriber function is ignored. Subscribers to the same event type are not guaranteed to be called in any particular order relative to each other.

All the concrete :app:`Pyramid` event types are documented in the :ref:`events_module` API documentation.

An Example

If you create event listener functions in a subscribers.py file in your application like so:

def handle_new_request(event):
    print 'request', event.request

def handle_new_response(event):
    print 'response', event.response

You may configure these functions to be called at the appropriate times by adding the following code to your application's configuration startup:

# config is an instance of pyramid.config.Configurator

config.add_subscriber('myproject.subscribers.handle_new_request',
                      'pyramid.events.NewRequest')
config.add_subscriber('myproject.subscribers.handle_new_response',
                      'pyramid.events.NewResponse')

Either mechanism causes the functions in subscribers.py to be registered as event subscribers. Under this configuration, when the application is run, each time a new request or response is detected, a message will be printed to the console.

Each of our subscriber functions accepts an event object and prints an attribute of the event object. This begs the question: how can we know which attributes a particular event has?

We know that :class:`pyramid.events.NewRequest` event objects have a request attribute, which is a :term:`request` object, because the interface defined at :class:`pyramid.interfaces.INewRequest` says it must. Likewise, we know that :class:`pyramid.interfaces.NewResponse` events have a response attribute, which is a response object constructed by your application, because the interface defined at :class:`pyramid.interfaces.INewResponse` says it must (:class:`pyramid.events.NewResponse` objects also have a request).